In conventional systems of this nature, a mechanical or electronic switch is periodically opened or closed under the control of an engine-driven shaft, e.g., by a cam on that shaft or by a photoelectric signal detector coupled therewith. In general, ignition should occur during the compression stroke of the reciprocating piston of each cylinder, prior to the arrival of that piston in the end position of that stroke (usually referred to as upper dead center or UDC). However, the extent of advancement of the firing point with reference to the UDC position for optimum efficiency depends on engine speed and is not constant in terms of either time or angle of rotation.
In earlier systems, the advance of the firing point was controlled mechanically through a linkage between a centrifugal speed sensor and the switch contacts of their controller. More recent developments, e.g., as disclosed in French Pat. No. 2,099,230, include electronic circuitry for establishing a variable delay between a predetermined reference point in the operating cycle, preceding the UDC position by more than the maximum advance of the firing point, and the instant of ignition. Many of these prior electronic systems were inherently unstable and required the establishment of at least two fixed reference points per cycle to provide a gauge for the requisite delay with continuous readjustment of that delay in response to an error signal.